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APPENDI X
D
A n In t rodu c t ion to Sup erpav e
by Kamyar C . Mahboub, Ph
.D
., P
.E
Superpave is a product of the Strategic Highway Research Program (SHRP)
. This re
search effort led to a new system for design of hot mix asphalt based upon mechanisti
c
concepts. The Superpave
T M
has been fully implemented by most of the state highwa
y
agencies
. Superpave is an acronym for Superior Performing Asphalt Pavements
. Th
e
Superpave system accounts for materials characteristics in light of climatic and traffi
c
considerations (AI, 2001, 1996, 1997) . Perhaps the most significant component of Su
perpave is its new asphalt binder grading system, which is designed to link with pave
ment performance . The Superpave methodology is believed to be the best available at
this time. However, it is an evolving methodology, and as such there are various asphal t
characterization routines that are under consideration as future additions to th e
Superpave (Witczak, et al
2002)
D 1
SPH LT BINDER GR DING SYSTE
M
The asphalt binder grading system in Superpave is called the performance gradin
g
(PG) system
. This system is a radical departure from the previous viscosity or penetra
-
tion based systems
. All PG binders are characterized based upon fundamental engi-
neering parameters . Additionally, Superpave accounts for the impact of climatic factor s
on binder characteristics at both hot and cold temperature regimes
. This is a major im
-
provement over previous systems of asphalt binder grading
. In addition to climati
c
conditions, traffic and aging control the performance of the asphalt pavement . To sim -
ulate climate conditions, testing is conducted at three pavement temperatures : hot, in-
termediate, and cold pavement temperatures
. These temperatures are derived fro
m
weather data for various geographical locations . The climatic data is further trans-
formed to represent the pavement temperature
. To simulate traffic conditions, an aver-
age rate of loading was assumed for normal highway traffic speeds
. Heavy traffi
c
conditions may be addressed by selecting a binder corresponding to higher tempera-
ture regimes. To simulate binder aging, a new rolling thin-film oven procedure wa
s
682
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D .1 Asphalt
Binder Grading System 68
3
C rite ria for The rm al C rite rion for
Criteria for
Criterion for
Cracking
Fatigue Cracking
Rutting
Workability
S (60s) <300MPa G*sins G*/sin6
Viscosity at 2
0
m (60s)>0
.300
<5
.OMPa
unaged>1
.OkPa
rpm < 3
.0Pa-se
c
Failure strain>0 .01
RTFO >2 .2kP
a
Pressure Ag ing Vesse l
Direct Tension
Thin Film O ve n
Residu
e
Intermediate
H ig h
Temperature
Temperatur e
D ynamic Shear
D ynamic Shea r
Rheometer Rheometer
Bending
Beam
Rheometer
-2
0
FIGURE D
A summary of mechanical tests related to asphalt binder PG grading
developed under Superpave, which allows for rapid aging/oxidation of an asphal
t
binder under simulated conditions
The Superpave binder grading tests are based upon engineering properties that
control three major modes of distress in asphalt pavements
: rutting, fatigue cracking
and thermal cracking. The contribution of the asphalt binder to these modes of distres
s
is characterized through a battery of rheological tests which are outlined in Figure D1
The test data are analyzed in light of climatic conditions for determining the asphal
t
binder grade . For example, a PG 64-28 is suitable for an environment where the maxi
mum pavement temperature will not exceed 64°C, and the minimum pavement tem-
perature will not drop below -28°C
.
Figure D
.1 presents a summary of mechanical tests related to asphalt binder PG grad
ing
. The direct tension (DT) test is intended to determine the resistance of asphalt to ther
mal cracking
. Similarly, the Bending Beam Rheometer (BBR) is designed to measure th
e
critical stiffness (S) at which the asphalt becomes brittle and susceptible to thermal crack
ing (m=slope of stiffness courve)
. In order to simulate the most severe case, the thermal
cracking analysis is conducted using the asphalt which has gone through the accelerate d
aging process using the pressurized aging vessel (PAV)
. The Dynamic Shear Rheomete r
(DSR) is the device that is used for fatigue and rutting characterization . The rheometer
protocols are designed to measure elastic and damping properties of the asphalt binde
r
via the complex shear modulus parameter (G*)
. The rutting parameter is G*/sin 8
Brookfiel d
Viscosity
Pavement Tem perature, °C
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68 Appendix D ntroduction to Superpav
where S is the phase angle and is related to damping . On the other hand, the G*sin S i
s
used to characterize the fatigue-cracking potential of asphalt
. The fatigue characteriza
tion is conducted on asphalt, which is aged via the PAV process, while the rutting char
acterization is conducted on the asphalt binder that is aged using a Rolling Thin-Fil
m
Oven (RTFO) test
. Superpave binder grading protocols are outlined in AASHT O
MP1 specifications (AASHTO, 1999a)
. Tables D
.1 through D .4 present Superpave Per
formance grade CPG binder specifications
D 2 AGGREGATES IN HM
A
Experience has shown that aggregates play a key role in HMA performance, and thi
s
was realized by SHRP researchers, which led to the refining of existing procedures t
o
fit within the Superpave system
. SHRP researchers produced an aggregate gradatio
n
specification without the benefit of experimentation to support or verify its formula-
tion . Thus, in lieu of experimentally verifiable protocols, a panel of SHRP experts de
-
veloped a set of recommendations for Superpave aggregate specifications (NCHR
P
Project 9-14,1997)
. This led to a number of controversial issues including flat and elon
-
gated aggregates, and the restricted zone
D 2
1 Flat and Elongated Aggregate s
The recommendation for flat and elongated aggregate content was that, for high traffi
c
(greater than 10
6
equivalent single axle loads—ESALs), no more than 10% of th
e
aggregate particles retained on the 4
.75 mm sieve should have a ratio of maximum-
to-minimum dimension greater than 5
:1 (Cominsky
et al
.
1994)
.
Vavrik et al
(1999) recommended performance based testing as a requirement t
o
establish if the use and breakdown of F&E particles had a detrimental effect on mix-
ture performance . Brown
et al
(1997) evaluated the effect of flat and elongated parti
cles on SMA mixes
. Stephens and Sinha (1978) studied the significance of flat an d
elongated particles on the characteristics of bituminous mixtures
. In a mix design, the
y
recommended 40-70 % cubical aggregates, 5-45 % flat aggregates and 5-45 To elon-
gated aggregates. Oduroh et al (2000) reported that coarse aggregates of 3
:1 size rati
o
at 40% and higher had the highest tendency to lie flat (horizontally) during HM
A
compaction . Overall, Superpave laboratory mixture performance tests did not sho
w
any significant changes in mixture properties due to the presence of up to 40% of 3
:
1
flat and elongated aggregates
D 2 2 Coarse Aggregate Angularit y
The aggregate interlock and internal friction is responsible for the HMA rutting resis
-
tance
. Aggregate angularity is quantified as the percent by weight of aggregates large
r
than 4
.75 mm with one or more fractured faces
. The standard test for measuring coarse
aggregate angularity is ASTM D 5821
:
Standard Test Method for Determining the Per-
centage of Fractured Particles in Coarse Aggregate
Superpave specifies a higher degre e
of aggregate angularity for higher traffic
. This is illustrated in Table D
.S
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Table D
.1 Superpave S pecifications for Ruttin g
Superpave Binder Specifications
Average 7-day Maximum Pavemen
t
Design Temperature, °C
Minimum Pavemen
t
Design Temperature, ° C
Flash Point Temp, T48
: minimum, ° C
Viscosity, ASTM D 4402 :
Maximum, 3-Pa-s (3000 cP)
Test Temp, °
C
Dynamic Shear, TP
5
G /sin S, Minimum, 1
.00 kPa
Test Temperature@ 10 rad/s,°C
olling Thin Film Oven (T240
)
Mass Loss, Maximum,
Dynamic Shear, TP5 :
G*/sin 8, Minimum, 2 .20 kP
a
Test Temp @ 10 rad/sec,°C
Table D
.2 Superpave Sp ecifications for Fatigue Cracking
Superpave Binder Specification
PAV Aging Temp,° C
Dynamic Shear, TP5
:
G*sin
8, Maximum, 5000 kP
Test Temperature@ 10 rad/s,°
C
Physical Hardening
Creep Stiffness, TP1 :
S, Maximum, 300 MP
a
m-value, Minimum, 0
.300
Test Temperature@ 60sec,° C
Direct Tension, TP3
:
Failure Strain, Minimum, 1 .0
Test Temperature@ 1 .0mm/min, °C
Superpav e
Specification Limit
s
for Rutting
Superpav
e
Specification Limi t
for Fatig u e
Cracking
68
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T able D
.3 Supe rpave Spe cifications for Thermal Crackin
g
Superpave Binder Specifications
PAV Aging Temp,°
C
Dynamic Shear, TP5
:
G*sin S, Maximum, 5000 kPa
Test Temperature@ 10 rad/s,°
C
Physical Hardening
Creep Stiffness, TP1
:
S, Maximum, 300 MPa
m-value, Minimum, 0
.300
Test Temperature@ 60sec,°
C
Direct Tension, TP3 :
Failure Strain, Minimum, 1
.0
Test Temperature@ 1
.0mm/min, °C
Table D .4 Superpave A sphalt Binder Grade
s
High Temperature Grades (°C)
Low Temp erature Grades (°C )
46
-34, -40, -4
6
5 2
-10, -16, -22, -28, -34, -40, -4
6
5 8
-16,-22,-28,-34,-4
0
64
-10,-16,-22,-28,-34,-40
70
-10, -16, -22, -28, -34, -4
0
76
-10,-16,-22,-28,-34
80
-10,-16,-22,-28,-3
4
Performanc
e
Grad
e
Designation :
Superpav
e
Specification Limit
s
for Therma l
Crackin
g
PG Hot Tem
p
Example : PG
64-22
Cold T em
p
686
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Aggregates in HMA 68 7
TABLE D
.5
Superpave Coarse Aggregate Angularit
y
Requirement
s
Traffic,
Minimum Fracture d
Surface Requirements (%
)
million ESALs D <
100
mm D
100 m
m
<
.3 55/
65 1
—
< 3
75/—
50/
<10
85/80
60/
<3
95/90
80/75
<100
100/100 95/90
100
100/100 100/100
Note. 85/80
means that 8 of the coarse agg regate has one frac-
tured face, and 80 has two fractured face s D
= depth from surfac
e
D 2
3 Fine Aggregate Angularit
y
Fine aggregate contribution to the internal friction of HMA is quantified as the per
-
cent of air voids present in loosely compacted fine aggregates (smaller than 2
.36 mm)
Higher void content in this case reflects a more textured fine aggregate . The standar d
test for measuring this property is AASHTO T 304
: Uncompacted V oid Content–Method
A
. Superpave specifies a higher degree of fine aggregate angularity for higher traffic
This is illustrated in Tables D
.6
D 2
4 Aggregate Clay Conten
Clay is a highly undesirable material in HMA
. The clay content is characterized via a
suspension in the water test
: AASHTO T 176 :
Plastic Fines in Graded A ggregates an
d
Soils by Use of the Sand Equivalent Test
The clay content is controlled using a mini
mum sand equivalent criteria
. Superpave requires higher sand equivalent (i .e . lowe r
clay content) for higher traffic
. This is illustrated in Table D.7
TABLE D .6 Superpave Fine Aggregate Angularit
y
Requirements
Traffic,
Minimum Uncompacted Fine
Aggregat e
Air Voids Requirements (%
million ESALs
D < 1 0 0 mm D >
100
m m
<0
3
—
<1
40 —
<3
40 40
<10 45 40
<30
45
40
<100
45 45
1 0 0
45 45
Note Air voids criteria are presente d as perce nt air voids i n
loosely compacted fine agg reg ate D = depth from surfac e
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688 ppendix D n Introduction to Superpav e
TABLE
D .7 Superpave Fine Aggregat
e
Angularity Requirement
s
Traffic, million
ESALs
Sand Equivalent, minimu
m
<0
.3 4
0
<1
4 0
<3
4 0
<10
4 5
<30
4 5
<100 5
0
100
5 0
D 2 5 Aggregate Toughnes s
Toughness is characterized using the Los Angeles Abrasion test
. The procedure i
s
described in AASHTO T 96
:
Resistance to Abrasion of Small Size Coarse Aggregate b
y
Use of the Los Angeles Machine . This test simulates the resistance of coarse aggregat e
to abrasion and mechanical impact during handling, construction, and in service . The
test is based upon comparing the coarse aggregate gradation before and after subject-
ing the aggregate to a mechanical degradation test
. The test measures the percent los
s
in the coarse aggregate
. The percent loss should be less than 35-45%
D 2 6 Aggregate Soundness
Soundness is the percent loss of materials from an aggregate blend during the sodiu
m
or magnesium sulfate soundness test
. The procedure is stated in AASHTO T 104
:
Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate
This test esti
-
mates the resistance of aggregate to weathering while in service . It can be performe
d
on both coarse and fine aggregate
. The test is performed alternately by exposing an ag-
gregate sample to repeated immersions in saturated solutions of sodium or magnesiu
m
sulfate each followed by oven drying. One immersion and drying process is considere
d
one soundness cycle
. During the drying phase, salts precipitate in the permeable voi
d
space of the aggregate
. Upon re-immersion, the salt rehydrates and exerts internal ex-
pansive forces that simulate the expansive forces of freezing water
. The test result i
s
the total percent loss over various sieve intervals for a required number of cycles
. Max -
imum-loss values range from approximately 10—20% for five cycles
D 2 7 Aggregate Gradatio n
Superpave aggregate gradation requirements posed several controversial issues . For ex-
ample, the initial versions of Superpave included a restricted zone in the gradation . Thi
s
zone was identified on a 0
.45-power gradation chart to define a permissible gradation . Th e
0
.45-power chart (Figure D2) is a common format for plotting aggregate gradation, be
cause it can easily illustrate the maximum density line as depicted in Figure D2 . It was ini -
tially hypothesized that gradations that violate the restricted zone possess weak aggregat
e
skeletons which may exhibit tenderness during construction and poor performance
. How
ever, recent research (TRB, 2002) suggests that Independent results from the literatur
e
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3 Asphalt Mix Design 68
9
0 .075 .3
6 1
18 2.3 6 4 .7 5 9 .5 12 .5 19 0
Sieve Size, mm Raised to 0
.45 Powe r
FIGURE D
2
0
.45-Power curve
clearly indicated that no relationship exists between the Superpave restricted zone an
d
HMA rutting or fatigue performance . This report further recommends that perhap s
all references to the restricted zone should be deleted from AASHTO MP2 (1999b )
and AASHTO PP28 (1999c)
The maximum density gradation represents a tight aggregate packing
. This typ
e
of gradation does not necessarily produce the best performing HMA
. Superpav
e
gradations are recommended to have a strong aggregate interlock, which is common i n
more open mixes
D 3 ASPHALT MIX DESIG N
The Superpave mix design is based upon mixture volumetric properties at a specifie d
level of compaction . These volumetric properties are assumed to produce well performin
g
mixtures (AASHTO MP2 : Superpave Volumetric Mix Design)
Advanced Superpav
e
protocols are available for mixture performance analysis
:
Standard Test Method fo
Determining the Permanent Deformation and Fatigue Cracking Characteristics of Hot Mi
x
Asphalt HMA) Using the Simple Shear Test SST) Device, AASHTO TP7 Provisional
Standards, 1998
The mixture compaction is accomplished via the Superpave gyratory compacto
r
(SGC), and the resulting volumetric properties are used to select the optimum asphal
t
content
. The Superpave volumetric terms are defined in Table D B
D 3 1 Superpave Laboratory Compactio n
The Superpave gyratory compactor (SGC) is designed to produce specimens in th
e
laboratory which exhibit particle orientation similar to the field compacted mixtures
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690 Appendix D
A n Introduction t o Superpav e
T A B L E D
Superpave Volumetric Terminology
Constituent Abbreviated Term
Paramete r
Aggregate P—mass percen
t
G—specific gravit y
s—stone (aggregate
)
b—bul
k
a—apparent
e — effective
Asphalt binder P—mass percent
G—specific gravit y
b—binder (asphalt
a— absorbed
e— effect iv e
Mixture
G—specific gravit
y
b—bul k
m — ma x imu m
m — m ixtur e
V—volume percent
Ps
—percent of mixture which is ston
e
Gsb
—bulk specific gravity of stone
G
s a
—apparent specific gravity of stone
G
S e
—effective specific gravity of ston
e
Pb
—percent of mixture which is binde
r
P e
—
P
ercent effective binde r
Pba — P
ercent binder absorbe d
Gb —specific gravity of binder
Gmb —bulk gravity of mi
x
G
mm —maximum theoretical gravity of mi
x
V
a
—volume of air in compacted mi
x
VMA—voids in mineral aggregat
e
VFA—voids filled with asphalt
D :B ratio—dust to binder ratio
This is achieved with a mold gyrating 30 revolutions per minute at 1
.25-degree pivo t
angle at the compaction pressure of 600 kPa
. The number of revolutions are adjuste d
to produce a target density.
There are a number of issues surrounding the Superpave laboratory com-
paction. The main concern is the relationship between laboratory and field com-
paction (Blankenship, et al
., 1994) . For example, Peterson et al
(2003) hav
e
suggested a number of modifications in order to improve upon the existin
g
Superpave compaction protocols
. These modifications include revisiting the angl e
of gyration and compaction pressure
. Additionally, there are standardization tool s
and techniques which are becoming available for the calibration of the angle o f
gyration
D 3
2 Mix Design Criteri
a
Once the proper aggregate and grade of asphalt binder have been selected, the nex
t
step is to produce a mixture that meets the Superpave criteria
. The goal of the mi
x
design process is to determine the optimum asphalt content corresponding to a set o
f
Superpave volumetric criteria . The most critical criterion is a 4% air content in the lab
-
oratory compacted specimens
. Additionally, voids in the mineral aggregate (VMA
)
and voids filled with asphalt (VFA) are checked
. Superpave volumetric characteristic
s
are determined at various compaction levels, which correspond to various traffic level
s
in the field
. AASHTO reports the required compactive effort for various traffi
c
(AASHTO, 1999b and 1999c)
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D 3 Asphalt Mix Design 69
The mixtures are compacted to N
i , N
and N
f
in accordance with the following :
N
;—Number of initial gyrations
: This parameter indicates a potential for tender-
ness in the mix during construction
N
—Design number of gyrations : The number of gyrations for which the mix i
s
designed to produce 4% air content
Nf—Final number of gyrations
: The final number of gyrations is designed to sim-
ulate the post-compaction densification due to traffic
. A mixture demonstratin
g
less than 2 % air content at this point would be susceptible to rutting
N„ —Maximum number of gyrations that should never be exceeded
The volumetric properties of trial batches at 0
.5% asphalt content increments ar
e
measured, and the optimum asphalt content is selected based upon an air content of
4
% (AASHTO MP-2)
Finally, the moisture sensitivity of the mixture is determined in accordance wit
h
AASHTO T283
. This test is designed to measure the effect of moisture and cycles o
f
freeze—thaw on a mixture containing 7 % air
. The indirect tensile strength test is use
d
to quantify the laboratory-simulated moisture damage
. A strength ratio of 80% or
higher is normally required
. A strength ratio below 80% hints at a potential suscepti-
bility to stripping
.
Example
A Superpave mixture was prepared using for t rial ba tches at 0
.5%
asphalt content increments
All volumetric and com paction data are presented in the tab le below
:
B atch 1 Batch
2
Batch
3
Batch
4
1-AC(%)
4 .5 5 .0
5 .5 6
0
2-
Air (% ) a t N d
6
1
4
1
3 .0 2 0
3 - G m m
2 .467
2.444 2 .430 2.410
4-
VMA ( )
15.6
15 .1 15 .2 15 3
5- VFA
(% ) 62.1
72 .7 81 .5 8 9 1
6 - % G m m
at N
i 8 4.1
86
1
87 .0 8 8 1
7 - % G m m
at N
m 95 .4 97
1
98 . 6 99 .5
olut on
Using an interpolation routine or a graphical solution will reveal that
5
.1% would b e
the optimum asphalt content at which the
4% air content requirement is sat isf ied. As a cros
s
check, all other volum etric propert ies are reviewed at this optimum asphalt content to ensur e
their compliance with Superpave requirements
SUMMARY
Superpave has put the asphalt mix design and analysis on a rational platform
. There
are many who may argue that Superpave is not purely mechanistic
. However, mos
t
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692 Appendix D
An Introduction to Superpav e
would agree that features such as the PG binder systems and mixture analysis proto-
cols, are major improvements over the empirical methods of the past
. The PG binde r
system incorporates the climatic information in the binder selection process
. The volu
metric properties of HMA are used for Superpave mix design
. Many of these proper
-
ties lend themselves to quality control measurements
Protocols related to Superpave aggregates were developed based upon literatur
e
review rather than sound experimentation
. This resulted in many early, less-than-per
fect aggregate specifications, which were later adjusted
. Additionally, there are ne
w
procedures being developed in order to do a better job with addressing modifie
d
binders, recycled asphalt, and waste materials in HMA
.
A mechanistic mix design methodology provides us with the opportunity to inte
grate asphalt mix design and flexible pavement structural design (Mahboub and Little
1990) . Superpave is hoped to link with the latest AASHTO pavement design guide in
a
mechanistic manner
P R O B L E M
S
D
What is Superpave? Why is it innovative ?
D 2
What is the PG system? Why is it considered an improvement over the viscosit
y
and penetration systems ?
What are the critical factors affecting asphalt performance in pavements
?
What are the engineering parameters used in rating the asphalt binders? Ho
w
are these parameters tested in Superpave
?
5
For an asphalt pavement, the maximum consecutive seven day pavement tem-
perature is 66°C and single event coldest temperature is -20°C . The desig
n
ESALs for this pavement is 10,000,000, and traffic speed is standard
. Select a P
G
grade for this pavement
.
6
Can Superpave be used for both base and surface mixes
?
Use the maximum gradation chart in Figure D
.3, and draw a typical Superpav e
surface mix gradation
D 8
List the steps in the Superpave mix design
.